Acoustic apparatus for motor driven vehicles



March 4, 1941. s U ETAL 2,233,459

ACOUSTIC APPARATUS FOR MOTOR DRIVEN VEHICLES Filed May 29, 1937 F .3. F a I G ia l Bnventors E'ederic E zS'hou/p Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE ACOUSTIC APPARATUS FOR MOTOR DRIVEN VEHICLES ware Application May 29, 1937, Serial No. 145,634

21 Claims.

Our invention relates to acoustic apparatus, and more particularly to an improved arrangement for installing sound reproducing devices in automobiles or other motor driven vehicles.

' Heretofore, in the installation of radio receiving apparatus, including loudspeakers, in automobiles, it has been customary to fasten the loudspeaker to the dash wall, sometimes called a fire wall, of the automobile. The acoustic reproduction has not been good, particularly in the low frequency end of the sound range, because the sound waves from the rear of the speaker have substantially cancelled those from the front.

It has been proposed to overcome this difliculty by cutting a large circular hole in the dash wall and mounting the speaker on the engine side of the wall with the front of the diaphragm adjacent the hole so as to transmit sound through the wall as a baffle, in accordance with old and Well known acoustic principles. See, for example, Rice 8: Kellogg paper on A New Type of Hornless Loudspeaker-A. I. E. E., September, 1925. While this arrangement has been quite satisfactory in the case of radio cabinets and in installations in living room walls in houses, it has not been found practical in automobile installations for the following reasons: There has been practically no available room on the engine side of the dash wall for mounting a speaker; the speaker would be subject to objectionable conditions of heat, moisture and dirt, etc.; there have been objections on the part of car manufacturers and owners to cutting a sufficiently large opening in the dash wall for a speaker, thereby weakening the wall, to say nothing of the diiiiculty of cutting the opening. Furthermore, there is a difficult problem involved in the proper support of a speaker in this manner. Most serious, however, such an arrangement provides an easy path for engine noises from under the hood, through the opening and speaker diaphragm into the passenger compartment, and the sound from the rear of the speaker is radiated externally of the car, adding to trafiic noises.

It is, accordingly, an object of our invention to provide a new and improved acoustic arrangement for mounting a loudspeaker, having a small housing, in an automobile passenger compartment, or other installations, wherein exist the problem of obtaining an acoustic baffle of sufiicient area while, at the same time, excluding objectionable noise from external sources.

It is a further object of our invention to provide a new and improved acoustic arrangement for mounting a speaker on a baflle, or wall, on the side of the useful sound radiation.

A still further object of our invention is to provide a new and simplified mechanical arrangement for mounting a loudspeaker in an automobile, and for protecting the speaker from adverse conditions of moisture, heat and dirt.

It is a more specific object of our invention to provide a universal mounting arrangement for a loudspeaker which is readily adapted either for mounting in the usual manner on the dash, without provision for eliminating back Wave interference, or for mounting in our improved manner.

Still further objects of our invention are to provide a loudspeaker installation in an automobile or the like, between the engine compartment and the passenger compartment, to obtain improved acoustic performance while first preventing the entrance of engine noises from the engine compartment into the passenger compartment, and secondly, reducing the undesired and objectionable radiation of sound into the street from the rear of the speaker, adding to traffic noises.

In the accompanying drawing, we have illustrated several embodiments of our invention, which we shall now describe in detail, so that those skilled in the art may understand the same. We desire to make it clear, however, that these constructions are merely practical examples and are not limitations of our invention. In the drawing,

Figure 1 is a partial view, in perspective, of the interior of an automobile looking toward the instrument panel and showing the installation of radio apparatus,

Figure 2 is a side elevational view, partly in section, of a dash wall of an automobile or the like, and a speaker mounted thereon,

hFigure 3 is a side elevational view, partly in section, of an automobile dash wall and a loudspeaker mounted thereon in accordance with our invention.

Figure 4 is a rear (engine compartment) view in elevation of an automobile dash wall provided with mounting and acoustic structure in accordance with our invention. Figure 5 is a side elevational view, partly in section, of the structure of Fig. 3, showing in enlarged form and in greater detail, features of our invention with modifications,

Figure 6 is a fragmentary side elevational view, in section, of a modification of our invention, and

Figure 7 is a slightly enlarged'side elevational and fragmentary view in section, corresponding to Fig. 5, embodying a further modification of our invention.

In accordance with our invention, we have constructed a loudspeaker with a small rear diaphragm enclosure or housing that is adapted to be mounted on the passenger side of an automobile dash wall; the rear enclosure of the housing is sealed acoustically from the front of the diaphragm to prevent the cancellation effect of low frequency sound waves from the rear of the diaphragm. We have provided a plurality of acoustic equalizing passages between the enclosure and the engine side of the dash wall, preferably in the form of apertures in the dash wall enclosed by the housing, that is sealed acoustically to the Wall at its rear end, to relieve the enclosed sound pressure which is considerable in the case of small enclosures, thereby enhancing the faithful reproduction of sound; the areas of the apertures are designed to give certain desired acoustic effects. An important advantage of the above arrangement is that the equalizing apertures cooperate with the sound compartment to the rear of the speaker diaphragm to form a lowpass filter to prevent the entrance of engine noise through the dash wall opening and speaker, and at the same time materially reduce the sound radiation from the rear of the speaker into the street. The term equalizing means that the sound pressure Within the enclosure is made more nearly equal to that at the rear of the wall.

We have found that the passenger compartment of a car corresponds acoustically to the inside of a radio cabinet, but that is more effective in low frequency response, having a low frequency peak around 60 cycles in the case of ordinary passenger cars. Hence it is desirable that the low-pass filter relieve the sound pressure in this region while preventing the passage of sound above, say, 200 cycles, in order to exclude engine noise and reduce the radiation of back Waves into the street, the waves of higher frequencies being most objectionable and those suppressed by our filter.

Referring to the drawing, Fig. 1 shows a loudspeaker I mounted on the vertical portion of the dash wall 3 of an automobile, and electrically connected by a cable 5 to a radio receiver 1 that is mounted under the instrument panel 9 of the automobile.

Referring to Fig. 2, we have illustrated a loudspeaker I with an annular sheet metal housing I I and rear sheet metal cover plate I3 provided with sound openings I5. The arrows show how sound waves are short-circuited from front to rear of the speaker by a short path, because of the small size of the speaker housing, with the resulting deficiency of low frequency sound radiation. For full and proper reproduction, the distance should be at least one-quarter the wave length of the lowest frequency sound waves desired, in accordance with the teachings of the Rice and Kellogg paper, above mentioned. The speaker is mounted on the dash wall 3 by means of a single hole and a. stud bolt I1 and nut I9 through a hole 20. The rear cover plate I3 is removable, as hereinafter described, although in practice the rear plate is ordinarily supplied for those who desire to mount the speaker as shown, with a sacrifice in acoustic performance.

Referring to Figs. 3 and 4, the rear cover plate of Fig. 2 has been removed, and the housing is mounted with its open rear end against the front (passenger compartment) surface of the dash wall 3. A gasket 2 I, preferably of soft rubber, is

'eter.

provided for acoustically sealing the housing to prevent escape of the rear sound waves, or, in other words, to prevent short circuit of sound waves of low frequency. In order to relieve the sound pressure, a considerable amount of which occurs when the rear of a speaker diaphragm is tightly enclosed by a small compartment, we have provided a plurality of sound passages, such as apertures 23, in the dash wall to the rear of the housing. In certain installations made in accordance with our invention, there were eight apertures about three-quarters of an inch in diameter arranged in a circle about seven inches in diam- The arrows illustrate the radiation of sound waves with the absence of the short-circuiting effect above referred to, the front and rear waves being isolated. With this arrangement, a service man may make the installation. It is only necessary to drill several small apertures, the above size being chosen in view of the availability of drills of this diameter.

Referring to Fig. 5, a loudspeaker of the per manent magnet dynamic type is shown within the housing I I, the front of which is covered with a grille 25, pervious to sound. The speaker per se comprises a metal dishpan or cone support 21 for a cone type diaphragm 29. The support 21 is provided with apertures 28 and is secured to the front of the housing II, as by means of bolts 2. The rear of support 21 is secured to the magnet structure 33, not shown in detail, and the bolt 41 is secured to the latter. When the nut I9 is tightened, the base of the housing II is drawn down snugly against the gasket 2| and dash wall, thus giving a very rigid and strong multi-point mounting, not affected by road vibrations or shocks. The dash wall 3 is ordinarily composed of sheet metal 2, and sound absorbing material such as a layer of hemp 4 and a fiber sheet 6. The thickness of the wall is usually of the order of one inch. Preferably, the automobile manu facturer may provide the apertures 23, complete or in knock-out form, in the metal sheet 2 of the Wall, and it is necessary then for the service man to cut through only the relatively soft layers 4 and 6.

For the purpose of keeping out rain and dirt, the apertures 23 may be covered with a dust screen, such as cloth 35, or fine metal gauze of the type disclosed by Scheldorf Patent 2,065,751, issued December 29, 1936. This material is preferably arranged so as not to unduly load the diaphragm. It may be disposed between the housing II and the wall 3, as shown. It will be noted that there is a space between the screen 35 and the wall 3 around the apertures 23. This gives greater screen area than if the screen tightly covered the apertures and materially reduces diaphragm loading. The loading is still further reduced if the screen is of cloth, instead of metal gauze, since the cloth is ordinarily free to vibrate with the sound waves. In some cases it may be desirable to provide means for stiffening the screen in accordance with the teachings of the said Scheldorf patent, to provide acoustic resistance.

For'further improvement in the filtering of sound, as well as excluding foreign particles, particularly dust and water, we have provided an external filter in highly simplified form, in the engine compartment over the apertures 23. This structure consists of a circular sheet metal disc 4| that may be flat or turned inwardly at the periphery. It is spaced a desired amount from the dash wall and a gasket 43 is preferably provided between the peripheral edge and the wall.

The disc is providedwith apertures 45 disposed in staggered relation with respect to apertures 23. Apertures 45 may be disposed in a circle in concentric relation with respect to apertures 23, about bolt I! as an axis, and the circle may be larger or smaller than that for apertures 23 or both, i. e., two circular rows. Smaller apertures 4'! are shown disposed in a circular path of a larger radius than that of 23 for sake of illustration. The foregoing arrangement provides a tortuous path for sound, as well as for foreign particles which it is desired to exclude. The disc is retained solely by a nut on bolt I1, thereby requiring no additional fittings.

As a further modification, the disc may be fiat and the gaskets 43 and the apertures 45 and 41 may be omitted, allowing the sound to escape around the periphery which is spaced from the wall. Certain features of the disc in Fig. 5 may be combined with that in Fig. 6. Instead of having its periphery free all around, the sheet may be made rectangular and a gasket provided at the top and bottom only, allowing the sound to escape laterally, the cross-section of the disc in Fig. 5 serving to illustrate this point.

A noise filter is constituted by the acoustic cooperation between the apertures 23 and the speaker compartment formed by the housing H, the dash wall 3 and the diaphragm 29. This compartment constitutes a compliance or capacitance while the apertures 23, particularly in view of their depth, constitute an inertance or acoustic inductive impedance, having a certain amount of resistance, thereby forming a low-pass filter section. The depth of the apertures may be increased if greater inertance is desired, by the use of well known means. The covering 35 over the apertures 23 adds resistance. Another filter section is formed by the external filter structure above described and the apertures 45 and 41. We are well aware that speaker filters have been known for some time, hence we do not feel that it is necessary to go into an extended discussion of acoustic filter action. Reference is made to the following patents disclosing acoustic filters and theory thereof: Carlisle 1,837,755 and Stewart 1,692,317. The cone dishpan 21 with apertures 28 forms a partition tending to set up another filter section, but the effect is reduced because it is desirable to make the apertures large so as not to unnecessarily damp the diaphragm.

By way of modification, as shown in Fig. '7, the portions 24 from apertures 23 are cut loose at the bottom sides from sheet metal wall 2, but retained at the top so that they may be outwardly bent, like shutters or louvres, to shed water, dirt and oil. This construction is especially useful where the installation of an external filter in the engine compartment is impractical because of expense or lack of space. With this arrangement. it is preferred that the apertures be made larger than in the case of Fig. 3, since the overhanging portion 24 restricts the sound opening. This provides a means for varying the area of apertures, if desirable to obtain acoustic adjustment. When the external filter 4| is used, its apertures 45 and 4? may be provided with the same shutter structure as just described.

It will be understood from the above that we havev provided a new and improved wall mounting for a loud-speaker, an arrangement that possesses both mechanical and acoustical advantages. The apertures in the wall not only serve as acoustic equalizers for relieving the sound pressure, but cooperate with the speaker compartment to constitute a two-way noise filter. Without this filtering action, the engine noises would readily enter the inside of the car through the dash wall speaker opening, and the sound from the rear of the speaker would be radiated into the street as an objectionable disturbance or nuisance.

It would ordinarily occur to those skilled in the art that the restricted sound outlet from the rear through apertures 23, used instead of a single large opening about the size of the diaphragm, would seriously damp the action of the diaphragm. We have found, however, as a result of experiments, that such is not the case. The use of a smaller number of apertures of the size shown would materially damp the diaphragm, although the use of a larger number would not give noticeably better diaphragm action. Curves plotted between acoustic damping and total aperture opening show that the rate of change in damping is great with an opening the size of one of the above apertures, but starts to flatten out with the total amount of rear opening shown. From the viewpoint of damping reduction, the total opening should be large, while for filtering purposes it should be small. We have found that a compromise is possible, as above pointed out, to satisfy both conditions. We prefer that the opening area be divided into a plurality of small areas, although it is within the scope of our invention to provide a small number of openings, or even one, of larger area.

Although we have shown and described certain specific constructions, we wish it understood that our invention is not limited to any of the details or applications set forth unless specified in the claims. Our invention has utility, for example, in auditoriums or residences, where it is advantageous to mount a speaker on a floor or a wall, with some or all of the functions and advantages above set forth.

While the terms baffle and wall are ordinarily considered to be a plane surface, there are directional bafiies, a species of horn, so that we, do not intend the use of such terms to be limited unnecessarily. In fact, in the illustration of our invention, the general arrangement of the front end of the passenger compartment, under the dash where the speaker is mounted, roughly resembles a directional baffle. In the application of our acoustic filter for the suppression of disturbing sound, it is not essential that the speaker be mounted, as shown, on the side of a bafile wall from which useful sound is radiated, nor is it necessary that the rear of the housing be sealed by means of the dash wall.

The basic principle of our invention may be carried out in various other constructions without departing from the scope of the invention as defined in the appended claims.

We claim as our invention:

1. In combination, a loudspeaker, an acoustic baffle, structure for mounting said speaker on said baflie on the front face thereof from which useful sound radiation is obtained, a housing annularly surrounding said speaker and sealed acoustically at its rear to said baffle, and means including a sound opening in said baflle for conducting sound waves, radiated from the back of said speaker, to the space behind the rear face of said bailie, said baffle being of sufiicient area to substantially prevent short circuiting low frequency sound waves.

2. The invention as set forth in claim 1 wherein said means comprises a restricted sound passage as an element of an acoustic filter.

3. The invention as set forth in claim 1 wherein said means comprises a plurality of restricted sound passages in said bafiie, said sound passages having a, substantial depth for providing increased inertance.

4. The invention as set forth in claim 1 where in said means comprises at least one restricted sound passage in said baiiie at the rear of said speaker, said passage having substantial acoustic inertance.

5. In combination with a motor driven vehicle having a dash wall separating the engine and passenger compartments, a loud speaker having a diaphragm and mounted in. a housing which is open at its front and rear ends, said housing mounted on said Wall and within said passenger compartment for radiation of useful sound waves from the front of said speaker diaphragmwithin said passenger compartment, the rear end of said housing lying next said wall, said wall having a restricted sound opening in the portion of said wall covered by said housing for conducting interfering back sound waves, radiated from the rear surface of said diaphragm, into said engine compartment.

6. The invention as set forth in claim 5 wherein said opening comprises an element of a lowpass acoustic filter for suppressing engine noises.

7. The invention as set forth in claim 5 wherein said opening comprises a plurality of restricted sound passages having substantial acoustic inertance.

8. The invention as set forth in claim 5 characterized in that said housing annularly surrounds said diaphragm and is sealed acoustically to said Wall.

9. The invention as set forth in claim 5 characterized in that a dirt and rain shield is operatively associated with said opening and disposed in the engine compartment.

10. In combination with a motor driven vehicle having a dash wall separating the engine and passenger compartments, a loud speaker having a diaphragm and mounted in a housing which is open at its front and rear ends, said housing mounted on said wall and within said passenger compartment for radiation of useful sound waves from the front of said speaker diaphragm within said passenger compartment, the rear end of said housing lying next said wall, said Wall having a restricted sound opening in the portion of said wall covered by said housing for conducting interfering back sound waves, radiated from the rear surface of said diaphragm, into said engine compartment, and a shield in said engine compartment disposed over said sound opening and in spaced relation thereto.

11. In combination, an acoustic baiiie, a loudspeaker mounted thereon for radiation of useful sound from one side thereof, a source of disturbing sound on the other side of said baffle, an acoustic filter arranged to conduct sound of certain frequencies from the rear of said speaker to said other side of said baflie and to prevent the passage of sound disturbance of other frequencies from said source through said speaker to the side of the baffle used for useful sound radiation.

12-. The invention as set forth in claim 11 characterized in that said filter is a low-pass filter enclosing the rear of said speaker.

13. In combination, an acoustic baffle, a loudspeaker, having a diaphragm, mounted and disposed on the front side thereof from which radiation of useful sound occurs, means forming an enclosure for the rear of said diaphragm comprising a low-pass acoustic filter section sealed to the rear of said speaker and to said baffle for equalizing the difference in low frequency sound pressure between the enclosure and the rear side of the baffle, and an additional filter section for said speaker mounted on the rear side of said baflie, said baffle being of sufficient area to we vent short circuiting of low frequency waves from the front to the rear of said diaphragm.

14. In combination, a large acoustic compartment adapted to accommodate persons, a loud speaker, having a diaphragm, mounted on a wall thereof for radiation of useful sound into said compartment, means comprising a low-pass acoustic filter forming an enclostu'e for the rear of said diaphragm, structure sealing said means to an opening in said wall, whereby low frequency sound waves are conducted from the diaphragm enclosure to the exterior of said compartment while preventing the passage of high frequency sounds from disturbing external sources into said compartment through said speaker.

15. In combination, a baffle, a loudspeaker, with a diaphragm, mounted on said baffle on the front side thereof from which useful sound is observed, a housing annularly enclosing said diaphragm and sealed acoustically at its rear to said baffle, a restricted sound opening through said baffle in the portion bounded by said housing comprising at least one acoustic equalizing passage for releasing sound pressure from within said housing to the rear side of said baffle, the total opening area being great enough so as not to substantially damp the movement of said diaphragm but small enough to provide a substantial amount of inertance.

16. The invention as set forth in claim 15 characterized in that a screen is disposed between said diaphragm and said opening.

17. The invention as set forth in claim 15 characterized in that a screen is disposed between said diaphragm and said opening and in spaced relation thereto.

18. The combination with a motor driven vehicle having a dash wall separating the engine and passenger compartments, of a loudspeaker, having a diaphragm, mounted on said wall and within said passenger compartment for radiation of useful sound waves from the front of said diaphragm into said compartment, a housing surrounding said diaphragm provided with a. sound opening from the front of said diaphragm into said compartment and constituting a sealed enclosure for the rear of the diaphragm to prevent back wave interference, a plurality of apertures in said wall communicating with said enclosure for equalizing sound pressure, and a shield in said engine compartment covering said apertures in spaced relation therefrom, said shield being provided with sound apertures out of line with those in said Wall.

19. In a loud speaker installation for automotive vehicles, in combination with a panel adapted for use in said vehicle and of sufficient area to act as an efficient baffle for a loud speaker, a loud speaker, a housing therefor and wholly containing said speaker, said housing being open for the passage of sound waves at both ends, means mounting said housing against said panel with an end of said housing in substantial air excluding contact with said panel, said housing being supported on said panel and Wholly to one side thereof, said panel being perforated in the area thereof covered by said housing with at least one perforation of sufiicient size to relieve acoustic loading, so that said panel acts as a baffle for said loud speaker.

20. In a loud speaker installation for automotive vehicles, in combination with a panel adapted for use in said vehicle and of sufiicient area to act as an efiicient bafile for a loud speaker, a loud speaker, a housing therefor and wholly containing said speaker, said housing being open for the passage of sound waves at both ends, means mounting said housing against said panel with an end of said housing in substantial air excluding contact with said panel, said housing being supported on said panel and wholly to one side thereof, said panel being perforated in the area thereof covered by said housing with at least one perforation of sufiicient size to relieve acoustic loading, so that said panel acts as a baffle for said loud speaker, and means located on the opposite side of said panel to prevent entrance of foreign material into said housing.

21. A structure as claimed in claim 19 including a sealing means interposed between said speaker housing and said panel.

FREDERIC F. SHOUP. CYRUS O. CAULTO'N. 

